Conventionally, drill rigs have utilized a cable handling system for transferring a tubular such as drill pipe or casing from a piperack adjacent the well to a mousehole or well bore for connection to a previously transferred tubular or drill string. A cable extends from the drill rig and is attached to the selected pipe or tubular on the pipe rack. The tubular lies in a generally horizontal position, box end forward, such that the box end of the pipe is initially pulled from the pipe rack by the cable up the catwalk of the rig and through the V-door to assume a substantially vertical position above the drill floor. The lower end is then placed into the mousehole or well bore for connection to the previously transferred pipe and the cable is disconnected.
There are disadvantages inherent in the conventional cable handling technique. The manual involvement of attaching the head of a cable to the tubular and the subsequent movement of the pipe during the transfer operation in the vicinity of a worker gives rise to dangerous working conditions and pipe handling is a major source of injuries on a drill rig, particularly in offshore drilling operations. Secondly, pipe and particularly casing, is expensive. As the tubular is transferred from the pipe rack to the drill floor utilising the cable, contact between the tubular and the catwalk or other portions of the rig is made which can cause damage to the tubular and affect the integrity of the connections between successive ones of the tubulars. This is particularly true where casing is involved.
Prior art apparatuses other than cable handling techniques for gripping a drill pipe and transferring the pipe from a horizontal position on the piperack to a vertical position above the drill floor are known. In some of such prior art apparatuses, pipe handling apparatuses provide pipe handling without the necessity of manual interaction in grasping the pipe or transferring the pipe to the rig. One such apparatus is disclosed in U.S. Pat. No. 3,633,771 to Woolslayer et al which teaches a drill string moved by a strongback having hydraulic grasping jaws mounted a distance apart which exceed the length of a single drill pipe. This apparatus is mounted to the drilling platform and is centered in the V-door of the rig.
A second apparatus is that disclosed in U.S. Pat. No. 4,834,604 to Brittain et al. This patent teaches a strongback which is connected to a one-piece boom, the boom being mounted on a base located adjacent the rig and operating directly through the V-door of the rig. The strongback transfers pipe through the V-door to a vertical position and raises or lowers the pipe so that connection between the pipe and the drill string can occur.
Other prior art used to transfer tubulars does not provide the conventional movement of the tubular box end forward and pin end down in the vertical position; that is, the tubular is moved and must be rotated such that the pin end is in a downwardly directed direction for attachment to the drill string. This may necessitate the design of a special structure for the rig or, alternatively, it may require that the rig structure be modified to accommodate the pipe handling system.
A disadvantage with all of the prior art set forth above arises when breakdown of the pipe handling apparatus occurs. In this event, the breakdown may terminate the installation of the drill pipe or casing since the conventional cable handling technique for tubular transfer cannot be used as a backup. The apparatuses utilised may obstruct the catwalk or otherwise require substantial modification to the rig in order to allow conventional cable operation after breakdown.